Single optical fiber connector

ABSTRACT

A connector for coupling a transmitting single optical fiber to a receiving single optical fiber in which the ends of the fibers are mounted in the respective ends of a bore in an alignment tube. The end faces of the fibers are spaced from each other and a restriction is formed in the bore between such end faces. The wall of the bore adjacent to each end of the restriction is tapered so that the optical beam emitted from the transmitting fiber is restricted in the restriction and thereafter reconstructed to its emitted numerical aperture prior to reaching the end face of the receiving fiber. The transmitting fiber is slidably mounted in the bore so as to be separable therefrom.

BACKGROUND OF THE INVENTION

The present invention relates generally to a connector and, morespecifically, to an optical coupler for single fiber optic cables.

The employment of fiber optic cables or light guides, also sometimesreferred to as optical communication fibers, for the tansmission ofinformation-bearing light signals, is now an established art. Muchdevelopment work has been devoted to the provision of practical low-lossglass materials and production techniques for producing glass fibercables with protective outer claddings or jackets. The jackets make themresemble ordinary metallic-core electrical cable upon superficialexternal inspection. Obviously, if fiber optic cables are to be used inpractical signal transmission and processing systems, practicalconnectors for the connection and disconnection of fiber optic cablesmust be provided.

Some references will now be given for background in the state of fiberoptic art in general. An article entitled, "Fiber Optics," by NarinderS. Kapany, published in Scientific American, Vol. 203, pgs. 72- 81,November 1960, provides a useful background in respect to sometheoretical and practical aspects of fiber optic transmission.

Of considerable relevance to the problem of developing practical fiberoptic connectors, is the question of transfer efficiency at theconnector. Various factors, including separation at the point ofabutment, and lateral separation or axial misalignment, are among thefactors effecting the light transfer efficiency at a connector. In thisconnection, attention is directed to the Bell System Technical Journal,Vol. 50, No. 10, December 1971, specifically to an article by D. L.Bisbee, entitled, "Measurement of Loss Due to Offset, and EndSeparations of Optical Fibers." Another Bell System Technical Journalarticle of interest appeared in Vol. 52, No. 8, October 1973, and wasentitled, "Effect of Misalignments on Coupling Efficiency on Single-ModeOptical Fiber But Joints," by J. S. Cook, W. L. Mammel and R. J. Grow.

Fiber optic bundles are normally utilized for only short transmissiondistances in fiber optic communications networks. On the other hand,fibers are used individually as optical data channels to allowtransmission over many kilometers. At present, most fiber optic cablesare multi-fiber bundles due to less stringent splicing requirementsgreater inherent redundancy and higher signal-to-noise ratio. Thedifficulty in achieving connections between single fibers which areinsensitive to axial misalignment problems has created an obstacle tothe use of long run single data transmission sytems.

Therefore, a connector or coupler is required to eliminate lateraltolerances if low-loss connections are to be obtained in the use ofsingle fiber optical transmission arrangements. "V" groove and metalsleeve arrangements have been used to interconnect single fibers.Reference is made to U.S. Pat. No. 3,768,146 which discloses a metalsleeve interconnection for single fibers. The problem in achievingalignment between single fibers is enhanced due to the typical lack ofconcentricity between the fiber core and its outside cladding or jacket.Thus, even if the optical fiber cables are perfectly aligned, the corestherein may be laterally displaced. Also, in typical single fibercoupling arrangements the end faces of the transmitting and receivingfibers abut one another, which may cause scratches on the end faces andthus light transmission losses. Further, if one fiber is slidablyinserted into the end of an alignment tube fixedly retaining the otherfiber, as is required for a separable connector, one or both of thefibers may fracture if axially compressed during the interconnection. Ifthe ends of the transmitting or receiving fibers were simply insertedinto the ends of an alignment tube, in axially spaced relationship, somelight transmission would be lost due to the fact that the receivingfiber would not completely fill the optical beam passing from thetransmitting fiber through the tube. If a shoulder were formed in thebore in the tube against which the receiving fiber could be positioned,light transmission losses between the outer surface of the fiber and thewall of the bore in the tube would be eliminated. However, it isvirtually impossible to maintain manufacturing tolerances such as toprevent the shoulder from extending over the peripheral region of thecore of the fiber, with the result that additional light transmissionlosses would be suffered. Therefore, what is needed and constitutes thepurpose of the present invention is to provide a coupling arrangementfor a pair of single optical fibers which may be incorporated into aconnector for practical field utilization, which is insensitive to fibercore alignment problems and problems in lack of concentricity betweenthe core and the cladding of the fiber cable, and in which the receivingfiber receives the entire optical beam transmitted from the transmittingfiber.

SUMMARY OF THE INVENTION

According to the principal aspect of the present invention, there isprovided a connector or coupler arrangement for coupling light from alight transmitting single optical fiber to a receiving single opticalfiber. The ends of the fibers are mounted in the respective ends of abore extending through an elongated alignment tube. The end faces of thefibers are spaced from each other in the bore. A restriction is formedin the bore between the end faces of the fibers. The wall of the boreadjacent to each end of the restriction is tapered. A reflective surfaceis formed on the wall of the bore between the opposed end faces of thefibers. One of the fibers, preferably the transmitting fiber, isslidably mounted in the bore so that it may be separable from thereceiving fiber. By the provision of the restriction in the bore, withthe tapered wall areas adjacent thereto, the optical beam emitted fromthe transmitting fiber is restricted and thereafter reconstructed to itsemitted numerical aperture prior to reaching the end face of thereceiving fiber. Such end face of the receiving fiber abuts against thetapered wall area adjacent to the restriction in the bore so that thefully reconstructed transmitted light beam impinges on the entire endface of the receiving fiber, so that light transmission losses in thecoupler are minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view through the connector of thepresent invention employing my novel optical coupling arrangement;

FIG. 2 is an enlarged fragmentary sectional view showing the details ofconstruction of the optical coupling arrangement illustrated in FIG. 1;and

FIG. 3 is a greatly enlarged fragmentary sectional view of the lighttransmitting area in the alignment tube of the coupler illustrated inFIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is generally applicable to the interconnection ofa pair of single optical fiber cables. A conventional single opticalfiber cable comprises an optical fiber and an outer protective jackettypically formed of plastic. The fiber consists of an inner core and anouter layer called a cladding. A variety of such optical fibers are nowavailable. The fibers may have a plastic core, with a plastic cladding,a glass core with a plastic cladding, or a glass core with a glasscladding. More recently, a chemical vapor deposition (CVD) fiber hasbecome available consisting of a quartz fiber having a germanium core.In the present invention, it is preferred than an optical fiber beutilized in which the cladding thereon may be readily removed. However,other forms of fibers in which the cladding cannot be removed may beutilized, but with greater light transmission losses.

Referring now to the drawings in detail, there is illustrated in FIG. 1a fiber optic connector in accordance with the present invention,generally designated 10, comprising a plug connector member 12 mated toa receptacle connected member 14. The plug connector member 12 comprisesa shell 13 containing a support member 16 which supports a plurality oftransmitting single fiber optic cables 18, only one being illustratedfor purposes of clarity. It is noted that the support member 16 containsfour axially extending passages 20 therethrough for holding the fiberoptic cables. It will be appreciated that any number of cables may bemounted in the connector member 12.

The mating receptacle connector member 14 also includes a shell 22containing a support member 24 for receiving single fiber optic cables26 equal in number to the cables 18 in plug connector member 12 andaxially aligned therewith. The shell 22 has a mounting flange 27 thereonfor mounting the connector 10 to a suitable panel or the like. The plugconnector member 12 carries a rotatable coupling nut 28 having anaccurate slot therein which cooperates with a pin 32 on the shell 22 toprovide a bayonet connection between the two connector members, as wellknown in the electrical field, which allows the two connector members tobe axially mated upon rotation of the coupling nut 28.

The support members 16 and 24 in the connector shells may be singlepieces or multiple piece arrangements, as illustrated in FIG. 1.

The transmitting fiber optic cable 18 comprises an optical fiber 34 andan outer plastic jacket 36. The fiber 34 consists of an inner core 38,which may be formed of plastic or glass, and an outer plastic cladding40. A termination pin 42 terminates the end of cable 18. The forward endof the pin extends beyond the front face 44 of the support member 16. Acylindrical recess 45 is formed in the forward end of the pin. The endof the plastic jacket 36 of the cable is cut and removed leaving a bareend portion 46 which extends into the recess 45 of the termination pin.An end portion of the plastic cladding 40 is also removed therebyleaving an unclad end of the fiber core 38 concentrically positionedwithin the cylindrical recess 45. It is noted that the front face 48 ofthe core 38 is positioned behind the forward end of termination pin soas to be entirely surrounded by the pin and thereby protected againstdamage.

A collar 50 is slidable on the pin 42 behind annular groove 52 in thepin. A spring retention element 54 has a pair of forwardly and inwardlyextending spring fingers 56 engaging rearwardly facing shoulder 58 onthe collar 50 limiting rearward movement of the termination pin in thesupport member 16. A resilient annular ring 60 lies within the groove 52for axial tolerance relief of the pin.

The receiving optical fiber cable 26 may be identical to the cable 18.The end of the plastic jacket 62 is removed leaving a bare end portion64 of the fiber 66 of the cable exposed. The end portion of the plasticcladding 68 of the fiber 66 is removed leaving an unclad end of the core70 of the cable. A termination pin 72 terminates the end of the cable26. The pin has a generally cylindrical configuration and is dimensionedto slidably fit within the cylindrical recess 45 in the termination pin42 on the end of the cable 18. A slidable collar 74 and spring retentionelement 76 identical to the collar 50 and retention element 54 areprovided for the termination pin 72. Further, a resilient annular ring78 lies within an annular groove 80 in the pin to provide for axialtolerance relief of the pin. The pin 72 is also formed with a forwardlyfacing tapered shoulder 82 which engages a rearwardly facing surface 84on the support member 24 to limit forward movement of the terminationpin into the receptacle connector member 14.

The termination pin 72 extends into a cylindrical cavity 86 which opensat the front face 88 of the support member 24. The cavity 86 is axiallyaligned with the termination pin 42 in the plug connector member 18 andis dimensioned to slidably receive the forward end of the pin thereinwhen the two connector members are mated together. The forward end 90 ofthe termination pin 72 terminates just short of the front face 88 ofsupport member 24. The pin 72 is concentric with respect to thecylindrical cavity 86. Thus, when the connector members 12 and 14 areinterconnected, the forward end of the termination pin 42 extends intothe cavity 86 and the forward end of the termination pin 72 extends intothe recess 45 in the pin 42.

An elongated generally cylindrical alignment tube 92 is mounted in theforward end of the termination pin 72. The tube may be formed with apair of axially spaced annular grooves 94 and 96. The regions indicatedat 98 and 100 of the termination pin may be deformed into the grooves 94and 96, respectively, to axially retain the alignment tube in the pin. Agenerally cylindrical bore 102 extends longitudinally through thealignment tube in axial alignment with the fiber cores 38 and 70 of thecables 18 and 26, respectively. The outer ends of the bore adjacent tothe opposite ends 104 and 106 of the alignment tube are taperedoutwardly as indicated at 108 and 110 defining lead-in entrances forguiding the fiber cores 38 and 70 into the opposite ends of the bore102. Preferably an inwardly extending flange 112 is formed on theforward end 90 of termination pin 72 extending over the end to 104 ofthe alignment tube. The flange 112 is beveled as indicated at 114 tofurther assist in guiding the core 38 into the alignment tube.

When the connector members 12 and 14 are fully interconnected, asillustrated in FIG. 2, the end face 48 of the fiber core 38 is axiallyspaced from the end face 116 of the fiber core 70 within the bore 102 ofthe alignment tube. As best seen in FIG. 3, a restriction 118 is formedin the bore 102 between the ends faces 48 and 116 of the transmittingand receiving optical fibers. The wall of the bore is tapered atopposite ends of the restriction, as indicated at 120 and 122. When thetermination pin 72 is being assembled to the receiving fiber cable 26during manufacture, the core 70 of the cable is inserted onto the bore102 of the alignment tube until the outer periphery of the end face 116of the core abuts against the taper 122 in the bore, thereby providingan annular light seal in the region 124 between the wall of the bore andthe fiber so that the optical beam passing from the transmitting fibercore 38 through the core 102 will be completely received by thereceiving optical fiber core 70. No light rays can pass between theouter surface of the fiber core 70 and the wall of the bore 102surrounding the fiber. Preferably, the length of the fiber core 30 ofthe transmitting optical fiber cable 18 is chosen so that when theconnector members are mated the end face 48 of the core 38 is spacedfrom the tapered wall portion 120 of the bore 102. The length of thebore between the taper 120 and the end 104 of the alignment tube issufficient to overcome axial tolerances in the length of the fiber.Preferably, the maximum length of the fiber core is sufficiently shortto assure that the fiber does not engage the tapered wall 120 of thebore which would otherwise result in the fiber being axially compressedduring the interconnection of the connector members with possiblefracture of the fiber occurring. However, the invention is not limitedto the foregoing arrangement wherein end face 48 of the fiber is spacedfrom tapered wall 120. It is possible that fracture of the fiber wouldnot occur when the end face, indicated in dotted lines at 48', lightlyengages tapered wall 120 since axial compression forces may beaccommodated by flexing of the core 38 within its loose cladding 40, andby the flexing of the entire fiber 24 within the outer jacket 36.

The wall of the bore 102 is coated with a reflective layer 126 whichmust extend at least between the end faces 48 and 116 of thetransmitting and receiving fibers. The reflective layer should extend atleast to the tapered end 108 of the bore to accommodate axial tolerancesin the length of the fiber core 38. For ease of manufacture, it ifpreferred that the entire inner surface of the bore 102 between the endsof the alignment tube be provided with the reflective layer. Thereflective layer may be any suitable metal plating on the wall of thebore, for example, gold, silver or aluminum. Gold is the preferredcoating material because of its high reflective properties. It is notedthat the metallic layer 126 is deformed at the region 124 of the bore102 thereby enhancing the light seal between the core 70 and wall of thealignment tube. An index matching fluid 128 may be provided in the bore102 between the end faces of the fibers 34 and 66, if desired.

The alignment tube 92 may be formed of any suitable material. Forexample, the tube may be formed of glass, as illustrated in FIGS. 2 and3. The glass tubing may be a thick walled capillary tube, such as aconventional thermometer tube. The tube is first cut to its desiredlength and then the ends of the bore in the tube are ground to providethe tapered entrances 108 and 110. The grooves 94 and 96 in the outersurface of the tube may be formed by heating the wall of the tube anddeforming the same by use of a suitable tool. The restriction 118 in thebore 102 in the tube may be formed by heating the intermediate region ofthe glass tube and either pulling the tube axially to reduce its crosssection, or compressing the heated region of the tube by the use of asuitable tool. Obviously other materials and techniques may be utilizedfor forming the alignment tube.

It will be appreciated that when the connector members 12 and 14 areinterconnected, coupling the transmitting and receiving optical fibersin the alignment tube 92, the optical beam emitted from the transmittingfiber is restricted to a concentric smaller diameter beam in therestriction 118 in the bore 102 in the tube. The beam is reconstructedto its emitted numerical aperture in the tapered region 122 of the bore102. The receiving fiber 70 totally and concentrically fills the beamdiameter, thereby collecting all the optical beam transmitted from thefiber 38. It will be appreciated that the slope of the tapered walls 120and 122 of the light transmitting bore 102 should be no greater than theangle of emission θ (see FIG. 3) of light emitted from the transmittingfiber. If the slope of the tapered wall areas of the bore were greaterthan θ, the angle of propogation of some of the light rays emitted fromthe transmitting fiber would be too high for transmission through thereceiving fiber.

By the way of example only, when using 6 mil diameter transmitting andreceiving optical fibers, the diameter of the cylindrical portions ofthe bore 102 may be 7 mils, the diameter of the restriction 118 may be 5mils and the slope of the tapered areas 120 and 122 of the wall of thebore 102 may be between about 3° and 5° for maximum light transmissioncharacteristics.

It will be appreciated from the foregoing that the propogating opticalbeam emitted from the transmitted optical fiber 38 will maintain adiameter which is equal to or less than the diameter of the receivingfiber core 70. The received beam will have a numerical aperture which isequal or less than the transmitted numerical aperture. The receivedoptical beam is generally concentric to the receiving fiber although itis not required that there be absolute concentricity between thetransmitting and receiving fibers due to the restricted configuration ofthe light transmitting bore between the end faces of the fibers.Further, the transmitting fiber core 38 preferably has axial clearancewithin the bore 102 so as to avoid compressive fracture of the fiberwhen the connector members 12 and 14 are interconnected. As explainedpreviously herein, it is preferred that the optical fibers employed inthe connector of the present invention have cladding thereon which isremovable. However, permanently clad fibers could be utilized, but witha resultant small loss in light transmission due to a small percentageof the optical beam emitted from the transmitting fiber impinging uponthe the end surface of the cladding of the receiving fiber.

What is claimed is:
 1. A fiber optic connector for coupling atransmitting single optical fiber to a receiving single optical fibercomprising:an elongated alignment tube having a bore therethroughextending to the ends of said tube; a restriction in said bore betweenthe ends of said tube; the wall of said bore adjacent to each end ofsaid restriction being tapered outwardly; a transmitting single opticalfiber having an end slidably mounted in one end of said bore and havingan end face adjacent to said restriction; a receiving single opticalfiber having an end mounted in the other end of said bore and having anend face closely adjacent to the tapered wall leading to the end of saidrestriction closest to said other end of said bore, said restrictionaxially spacing said end faces of said fibers apart in said tube; and areflective surface on the wall of said bore between the opposed endfaces of said fibers.
 2. A fiber optic connector as set forth in claim 1wherein:said restriction with the tapered wall areas leading thereto hasa generally symmetrical configuration.
 3. A fiber optic connector as setforth in claim 1 wherein:the periphery of said end face of saidreceiving fiber abuts said tapered wall area of said bore closest tosaid other end of said bore to provide a light seal therebetween.
 4. Afiber optic connector as set forth in claim 1 wherein:said end face ofsaid transmitting fiber is spaced from the tapered wall area of saidbore closest to said one end of said bore.
 5. A fiber optic connector asset forth in claim 2 wherein:the periphery of said end face of saidreceiving fiber abuts said tapered wall area of said bore closest tosaid other end of said bore to provide a light seal therebetween; andsaid end face of said transmitting fiber is spaced from the tapered wallarea of said bore closest to said one end of said bore.
 6. A fiber opticconnector as set forth in claim 1 wherein:said reflective surface on thewall of said bore extends behind said end face of said transmittingfiber.
 7. A fiber optic connector as set forth in claim 1 wherein:theends of said bore adjacent to said ends of said tube taper outwardlyproviding lead-in entrances for said fibers into the ends of said bore.8. A fiber optic connector as set forth in claim 7 wherein:saidreflective surface on the wall of said bore extends at least between theinner end of said lead-in entrance adjacent to said one end of said boreto the larger end of the tapered wall area closest to said other end ofsaid bore.
 9. A fiber optic connector as set forth in claim 1wherein:said ends of said fibers are unclad.
 10. A fiber optic connectoras set forth in claim 1 wherein:the slope of said tapered wall areas ofsaid bore is no greater than the angle of emission of light emitted fromsaid transmitting fiber.
 11. A fiber optic connector as set forth inclaim 1 including:first and second mating connector members; saidtransmitting fiber being mounted in said first connector member; saidalignment tube and receiving fiber being mounted in said secondconnector member; and said end of said transmitting fiber slides intosaid one end of said bore in said alignment tube when said connectormembers are mated.
 12. A fiber optic connector as set forth in claim 11including: a termination pin in said second connector member surroundingsaid alignment tube and fixed thereto.
 13. A fiber optic connector asset forth in claim 12 wherein:the forward end of said termination pinextends into a forwardly opening recess in said second connector member,the wall of said recess being spaced from said pin to define an annularspace therebetween; and a second termination pin in said first connectormember surrounds and is spaced from said transmitting fiber, said secondtermination pin being slidably received in said annular space when saidconnector members are mated, and slidably receiving therein saidfirst-mentioned termination pin.
 14. A fiber optic connector forcoupling a transmitting single optical fiber to a receiving singleoptical fiber comprising:an alignment device having a bore therethrough;a transmitting single optical fiber having an end extending into one endof said bore; a receiving single optical fiber having an end extendinginto the other end of said bore; said fibers having opposed end faces insaid bore axially spaced from each other; and restriction means in saidbore between said end faces, said restriction means restricting theoptical beam emitted from said transmitting fiber and reconstructingsaid beam to its emitted numerical aperture prior to reaching said endface of said receiving fiber.
 15. A fiber optic connector as set forthin claim 14 wherein:one of said fibers is slidably mounted in said boreand, therefore, is removable therefrom.
 16. A fiber optic connector asset forth in claim 1 wherein:said fibers abut said tapered wall areas ofsaid bore, respectively.
 17. A fiber optic connector as set forth inclaim 1 including:an index matching fluid in said bore between said endfaces.